The present invention relates generally to devices for transmitting electromagnetic signals of a desired frequency band and more particularly to devices for transmitting electromagnetic signals of a desired frequency band which are designed to deflect electromagnetic energy which falls outside of the desired frequency band.
Coaxial electric devices, such as coaxial cables, coaxial connectors and coaxial switches, are well known in the art and are widely used to transmit electromagnetic signals between a source and a load. Coaxial electric devices are typically designed to transmit electromagnetic signals over 10 MHz with minimum loss and little or no distortion. As a result, coaxial electric devices are commonly used to transmit and receive signals used for broadcast, cellular phone, GSM, data and other uses.
A coaxial electric device typically comprises an inner signal conductor which serves to transmit the desired communication signal. The inner signal conductor is separated from an outer conductor by an insulating material, or dielectric material, the outer conductor serving as the return path, or ground, for the communication signal. The relationship of the diameters and the dielectric material properties of the components defines the characteristic impedance of the coaxial device. Such an electric device is referred to as coaxial because the inner and outer conductors share a common longitudinal axis.
It has been found that, on occasion, undesirable electromagnetic signals which fall outside of the desired frequency band are transmitted through coaxial electric devices. As an example, coaxial electric devices are susceptible to having naturally created, low frequency electromagnetic impulses (e.g., of the type produced by lightning) pass therethrough. As another example, coaxial electric devices are susceptible to having transient, large current, artificially created electromagnetic impulses (e.g., of the type produced by motors, switches and certain types of electrical circuits) pass therethrough.
As can be appreciated, the passing of undesirable electromagnetic signals through a coaxial electric device can potentially damage, or even destroy, the load which is connected to said coaxial electric device, which is highly undesirable.
As a result, it is well known in the art for coaxial electric devices to include some type of protective device for eliminating or deflecting these types of undesirable electromagnetic impulses before said impulses are transmitted to the load.
In U.S. Pat. No. 5,764,114 to G. Kühne, there is disclosed an electromagnetic pulse (EMP) filter which can be used simultaneously for a plurality of frequency bands which includes a housing mounted in the outer conductor and a λ/4 short-circuiting conductor, which is connected in an electrically conductive fashion to the inner conductor of a coaxial line and is connected in an electrically conductive fashion to the end face of a housing. Arranged between the housing and the short-circuiting conductor is at least one sleeve which is connected to the latter in an conductive fashion. The length of the short-circuiting line corresponds to the λ/4 length of the lowest frequency band transmitted. Considered together, the sleeves produce a number of cavity resonators which are connected in series and are tuned with their length to various midband frequencies. It is directly possible by means of such cavity resonators connected in series to transmit a plurality of frequency bands, and thus to protect terminals against damaging current surges of other frequencies not within these bands.
In U.S. Pat. No. 6,101,080 to G. Kühne, there is disclosed a de-coupled EMP-charge eliminator device in a co-axial cable. The device includes a conductor which connects to the internal conductor of the coaxial device and extends through a housing that is attached to the outer coaxial conductor. At the conductor end opposite the coaxial center conductor, there is a concentrated capacitance connected between the housing and conductor which becomes an RF short circuit, so that the conductor acts as a lambda/4 short circuit conductor. After this concentrated capacitance, an EMP charge eliminator device is connected from the conductor to the housing.
Although useful and well known in the art, coaxial electric devices of the type described above which comprise a protective device for filtering undesirable electromagnetic impulses traveling therethrough suffer from some notable drawbacks.
As a first drawback, coaxial electric devices of the type described above utilize a shunt conductor which is coupled to and extends orthogonally away from the inner conductor, the shunt conductor requiring a separate enclosure which extends out from the outer conductor at a right angle relative to the inner conductor, thereby significantly increasing the overall size of the device, increasing the manufacturing costs associated with manufacturing the device, and rendering the device difficult to mount onto certain enclosures, which is highly undesirable.
As a second drawback, a coaxial electric device of the type described in U.S. Pat. No. 6,101,080 utilizes a concentrated capacitor grounding component which is fragile and difficult to assemble, thereby increasing manufacturing costs, which is highly undesirable.
As a third drawback, it has been found to be relatively difficult to adjust the desired frequency band to be transmitted by the coaxial electric devices described above. In fact, in order to alter the desired frequency range to be transmitted through the central conductor, coaxial electric devices of the type described above require the manufacturer to use a multitude of different lengths of orthogonal housings and/or shunt components, which is highly undesirable.
As a fourth drawback, the multiple tube coaxial electric device described in U.S. Pat. No. 5,764,114 provides multiple resultant bands of operation which are too narrow for many applications. In addition, it has been found to be extremely difficult to simultaneously tune the multiple tubes in order to widen the performance of said device.
As a fifth drawback, each of the coaxial electric devices described above is provided with a single protective component which has a limited lifetime. As a result, the single protective component has been found, in time, to fail which, in turn, requires expensive replacement and/or repair, which is highly undesirable.
In U.S. Pat. No. 6,236,551 to J. Jones et al., there is disclosed a surge suppressor device for protecting hardware devices using a spiral inductor (hereinafter referred to as the Jones patent). The surge suppressor protects hardware devices from electric surges by isolating the radio frequency from an inner conductor. The surge suppressor includes a housing, an inner conductor, a surge blocking device, and a spiral inductor. The surge blocking device is inserted in series with the hardware devices for blocking the flow of electrical energy therethrough. The spiral inductor is coupled to the surge blocking device and is shunted to ground for discharging the electrical surge.
Although useful and well known in the art, surge suppressor devices of the type described in the Jones patent suffer from a couple notable drawbacks.
As a first drawback, surge suppressor devices of the type described in the Jones patent have significant geometry changes on the length of the center pin, notably the large diameter increase for the surge blocking discs and the spiral inductor. These large changes in the center pin RF impedance must be compensated for in the ID of the outer housing. Thus changing frequency requires re-tuning of the compensation geometry, which is relatively difficult.
Another more serious drawback is that the non-constant impedance of the center conductor makes use of compensated quarter wave principles, for predictable wide-band performance, difficult or impossible.
In U.S. Pat. No. 5,982,602 to R. L. Tellas et al., there is disclosed a surge protector connector (hereinafter referred to as the Tellas patent). The surge protector connector comprises a surge protector having a front plate, a rear plate and a hollow cylindrical body bridging the front and rear plates. A coaxial cable connector interface extends from the front plate, the connector interface being constructed and arranged to detachably engage with a mating coaxial cable connector at the end of a first coaxial cable. A cable attachment interface extends from the rear plate, the cable attachment interface being constructed and arranged to attach directly to a prepared end of a second coaxial cable free of another coaxial cable connector interface. The surge protector further includes coaxial inner and outer conductors extending through the hollow cylindrical body and extending between the cable attachment interface and the coaxial cable connector interface. The surge protector includes a curvlinear quarter-wavelength shorting stub having a first portion extending in a generally radial direction from the inner conductor through a gap in the outer conductor and a second portion extending in a generally annular direction circumscribing the outer conductor between the outer conductor and the cylindrical body.
Although useful and well known in the art, surge protector connectors of the type described in the Tellas patent suffer from a couple notable drawbacks.
As a first drawback, surge protector connectors of the type described in the Tellas do not readily allow for adjusting bandwidth frequency performance.
As a second drawback, surge protector connectors of the type described in Tellas which include a curvlinear shorting stub often experience problems due to the considerably sharp bend at the juncture between the radially extending first portion and the annularly extending second portion. Specifically, the initial radial direction of the first portion results in a smaller bend radius at the transition with the second circumferential portion. This smaller bend radius increases the forces of high current transients which, in turn, can deform or break the shorting stub, which is highly undesirable.
As a third drawback, surge protector connectors of the type described in Tellas include an outer conductor which includes a relatively large sized gap through which the shorting stub extends. As can be appreciated, the large size of the gap in the outer conductor limits the optimization of the outer conductor for RF performance or transient impulse application, which is highly undesirable.
As a fourth drawback, surge protector connectors of the type described in Tellas which include a shorting stub which is directly connected to the outer conductor do not allow for the pass-through of direct current voltage on the center conductor.